Semiconductor manufacturers are becoming more and more concerned about a phenomenon known in the industry as stress notching or metal notching in conductive lines. While stress notching is not completely understood, it is believed that notching in conductive lines occurs as a result of high stresses put on conductive lines by overlying oxide and passivation layers. Notches, which may also be considered as voids on the exterior surface of a conductive line, are believed to form in conductive lines to relieve points of high stress concentration. Aluminum and aluminum alloy lines, metals commonly used in semiconductor devices, are particularly susceptible to notching due to the high degree of thermal expansion coefficient mismatch between aluminum and surrounding insulating layers and due to the relatively low melting point of aluminum. The thermal expansion coefficient mismatch implies that each time the temperature of a semiconductor device changes, stresses are induced in aluminum lines because aluminum's thermal expansion coefficient is higher than that of surrounding materials. In order to relieve these stresses, aluminum atoms migrate, thereby forming stress notches. Because aluminum has a low melting point, the atoms can easily migrate, especially at relatively low elevated temperatures. Atom migration to relieve stress is also referred to as creep. Not only is stress notching, or creeping, temperature dependent, but also time dependent. The lower the temperature, the longer it takes for a notch to form. More information relating to stress notching can be found in an article entitled "The Influence of Stress on Aluminum Conductor Life," by T. Turner et al., which appeared in The Proceedings of the IEEE International Reliability Physics Symposium, 1985, pp. 142-147. Further information on the effects of stress on metal lines can be also be found in an article, entitled "Line Width Dependence of Stresses in Aluminum Interconnect," by R. Jones, The Proceedings of the IEEE International Reliability Physics Symposium, 1987, pp. 9-14.
Several proposals to reduce stress notching and to reduce the effects of stress notching are known. One approach is to change the material used for conductive lines to one which is less susceptible to metal notching, for example copper or tungsten. Using copper for metal interconnect lines has several problems, one being that copper is very difficult to etch. Another problem in using copper is that the adhesion between copper and overlying oxide and passivation layers is poor and can pose reliability concerns. While adding copper to aluminum conductive lines may reduce notching, beyond a certain copper content, device performance begins to degrade. Furthermore, as conductive line geometries continue to shrink, the addition of copper in aluminum lines seems to prove less effective. The use of tungsten for conductive lines has also been examined, and is implemented in some instances. A disadvantage in using tungsten in place of aluminum or aluminum alloys is that tungsten has a higher resistivity, therefore signal speed is reduced.
Another approach to solving stress notching is to form a cap on the conductive lines. Caps can be formed from titanium nitride, tungsten, or titanium-tungsten compounds. These materials have higher melting points than aluminum and therefore have a higher resistance to stress notching. A disadvantage in using these caps is that at least one additional process step is required to form the caps. Yet another approach in reducing notches is to reduce the stresses created by oxide and passivation layers which overlie conductive lines. Experimentation has been performed in this area, for example by varying the rate, temperature, and/or pressure at which these layers are deposited and by varying the chemical composition of these layers, yet much remains to be understood about stresses in oxide and passivation films.
Therefore, for the reasons discussed previously, a need exists for an improved semiconductor device which has reduced effects of stress notching in conductive lines, and more specifically for such device to also maintain existing device performance levels without requiring additional processing steps.